Influence of thickness of MgO overlayer on the properties of ZnO thin films prepared on c-plane sapphire for H2 sensing

ZnO/MgO double layered thin films were prepared on Al2O3 substrate by direct current (dc) sputtering at a low power of 40 W. The effect of thickness of MgO overlayer on the properties of ZnO films was investigated. X-ray diffraction patterns revealed that the crystallinity of ZnO films can be improved by addition of a MgO overlayer. The blueshift in the PL peak suggests that the optical bandgap of ZnO can be tuned by varying the thickness of MgO overlayer. SEM micrographs revealed that the surface morphology changes from uniform to dense porous and nano-flake like structure with increase in thickness of MgO layer. Pure ZnO and optimized ZnO/MgO films were tested for their sensing performance towards H2 at different temperatures. The maximum response of ZnO and ZnO/MgO sensor was observed to be 135 and 260, respectively, at an operating temperature of 140 °C. The very high sensitivity at such a low operating temperature is associated with the fast recovery time of 120 s, which could be achieved due to the promoting effect of MgO on ZnO.